The invention relates to a high-pressure discharge lamp comprising a discharge envelope or arc tube provided with two electrodes between which a discharge path extends, this lamp further being provided with a cylindrical shield near an end of the discharge envelope around and at a certain distance form the discharge envelope.
A lamp of the kind mentioned in the opening paragraph is known from U.S. Pat. No. 4,173,728. Cylindrical heat shields around the discharge envelope near an end thereof are known means for influencing the temperature of the end of the discharge envelope, more particularly in lamp types in which the lamp envelope contains an excess of a filling constituent. In such a situation, the temperature of the constituent present in excess in fact determines the vapor pressure of this constituent in the discharge envelope.
In general, such cylindrical shields are made of metal, such as Ta, Nb, Mo, because of the high resistance to heat and the high reflective power. Such shields are generally provided on or against the wall of the discharge envelope either directly or indirectly by means of a heat-conducting intermediate layer. It is then found that the temperature control very strongly depends upon the presence or absence of a good mechanical contact between the shield and the discharge envelope throughout the envelope circumference, which results in that in practical lamps a high degree of reproducibility of the temperature control and hence of the temperature adjustment is hardly possible.
The U.S. Pat. No. 4,173,728 provides a solution in which the cylindrical shield is arranged at a certain distance from the discharge envelope throughout its area. In this manner, a substantially equally effective heat reflection is obtained in combination with a high degree of reproducibility with respect to the temperature control to be attained. In the known solution, the cylindrical shield is directly secured to a rigid current conductor. Experiments have shown that during operation of the lamp this gives rise to migration of filling constituents through the wall of the discharge envelope. The phenomenon of migration has a detrimental effect on lamp properties, such as variation of the color point of the emitted radiation and increase of the arc voltage, and mostly results in shortening the life of the lamp.